b is short for bag, the verb. bag is used to mean that someone is going to hook up with someone else. so when someone is trying to bag someone else, as in they are flirting or starting to make moves, one would make his/her right hand into a b (thumb straight up, fingers curled behind curled pointer finger) behind the person who is trying to bag. in addition, someone could say "b" or type it on facebook, text it, etc.

Example 1: Jack goes over to Anna and says, "Nice shirt," in a flirty way.
Aaron goes behind Jack and puts up a b with his hand.
People laugh

Example 2: On facebook, Jamie writes "wish i could've seen you today :(" on Ryan's wall.
Adrian comments on the wallpost saying "b"